Non-invasive temperature measurement of frozen/near frozen organic matter using single-frequency microwave radiation is well documented. Such techniques are particularly useful in the transport and storage chain between producer and consumer to monitor the temperature of frozen produce. Standard operating practices and regulatory/legislative requirements in the food industry require frozen produce, such as meat, to be frozen below a prescribed temperature before and during shipment along the length of the cold chain. Arrival of insufficiently cooled produce at the recipient destination casts doubt on temperature maintenance (and measurement procedures) of each stage in the chain. To limit such liability, the various parties in the cold chain traditionally undertake random monitoring tests, typically via an invasive process of drilling the produce and manually inserting a temperature probe.
Such techniques are clearly unsatisfactory as they are:                Time-consuming and labour intensive,        Potentially inaccurate unless performed carefully,        Unable to check every item and thus problem produce may escape detection,        Non-automated, and        Wasteful; the tested produce typically requires re-sealing, re-packing, down grading or disposal.        
Microwave temperature measurement permits a rapid, non-invasive, potentially automated method of testing all the frozen produce supplied, as described in the applicant's earlier patent applications including PCT/NZ03/00279 incorporated herein by reference. Nevertheless, known microwave temperature measurement techniques still suffer from disadvantages, primarily related to the intrinsic physical characteristics of microwave interactions with ice and water.
The passage of microwave radiation through a given material is attenuated according to a function dependent on both material temperature and the microwave frequency. Moreover, the microwave attenuation diminishes significantly at sub-freezing point temperatures resulting in the frozen material effectively becoming transparent to microwave radiation.
This disproportional attenuation of microwaves by water rather than ice is observed in domestic microwave ovens during heating of frozen food. The outside layer of ice slightly absorbs microwaves and melts to form water which consequently absorbs almost all the subsequent microwave energy leaving little energy to heat the internal ice. The outer layer of the food is thus left cooked, while the centre remains frozen.
Furthermore, it has been determined that for the temperatures used in the food chain (ranging from 0° C. to −30° C.), the ratio of the water and ice composition (or ‘ice fraction’) present in meat or other similarly water-rich organic produce is constant for a given temperature. Combining the two properties (i.e. the disproportionate transparency to microwaves of ice compared to water and the correlation of the ice fraction and temperature) provided the basis for a non-invasive temperature measurement device utilising a single microwave frequency.
Such systems are capable of measuring the temperature of a single frozen meat carton, for example, by measuring the unabsorbed energy of a single microwave frequency passed through the meat carton and correlating with remaining unfrozen water to calculate the temperature.
However, the typical microwave frequencies (e.g. 2.4 GHz) employed in such systems cannot penetrate more than approximately 200 mm of water. Thus for meat cartons or the like, this restricts instantaneous measurements to single cartons. This poses disadvantages in measurement throughput as it would be typically desirable in most meat processing plants and subsequent stages in the cold chain to measure a whole pallet of cartons without un-stacking the individual cartons.
The electromagnetic energy attenuated by a material is given by the formula:Pz=Poe−2αz  (1)where P is the residual power                α is the attenuation factor (frequency and material dependent)        o indicates the initial condition, and        z indicates the equivalent depth of material (i.e. excluding any voids and non-material spaces)Thus, it can be seen that simply increasing the power transmitted does not permit thicker layers of water/ice to be measured, although it does impact directly on the noise level and signal resolution.        
Known microwave temperature measurement techniques are thus limited in their applications by thickness of the measured object. As previously discussed, this means for example that a pallet of cartoned frozen meat would require un-stacking before measurement of the individual cartons.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.